U.S. patent number 6,928,333 [Application Number 09/387,174] was granted by the patent office on 2005-08-09 for scheduling method for automated work-cell transfer system.
This patent grant is currently assigned to Advance Micro Devices, Inc.. Invention is credited to Michael R. Conboy, Elfido Coss, Jr., Patrick J. Ryan.
United States Patent |
6,928,333 |
Conboy , et al. |
August 9, 2005 |
Scheduling method for automated work-cell transfer system
Abstract
According to an example embodiment, the present invention is
directed to a new and efficient method for bringing at least two
items together from independent locations via separate paths in a
computer controlled manufacturing environment. Using the computer,
the probabilities for pickup and delivery of each of the two items
are generated and used to determine an efficient manner in which to
bring the items together via the separate paths.
Inventors: |
Conboy; Michael R. (Austin,
TX), Ryan; Patrick J. (Eugene, OR), Coss, Jr.; Elfido
(Austin, TX) |
Assignee: |
Advance Micro Devices, Inc.
(Sunnyvale, CA)
|
Family
ID: |
34806825 |
Appl.
No.: |
09/387,174 |
Filed: |
August 31, 1999 |
Current U.S.
Class: |
700/112;
700/99 |
Current CPC
Class: |
G06Q
10/06 (20130101); Y02P 90/86 (20151101); Y02P
90/80 (20151101) |
Current International
Class: |
G06F
19/00 (20060101); G06F 019/00 () |
Field of
Search: |
;700/99,100,106,112,115,116,121,213,214,95 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Picard; Leo
Assistant Examiner: Cabrera; Zoila
Parent Case Text
RELATED APPLICATIONS
The present invention is related to U.S. patent application Ser.
No. 08/990,059, now U.S. Pat. No. 6,108,585 entitled "PROBABILISTIC
DISPATCHING METHOD AND ARRANGEMENT" filed on Dec. 12, 1997, and
U.S. patent application Ser. No. 09/387,613, now U.S. Pat. No.
6,338,005 entitled "MANAGING TEST MATERIAL IN AN AUTOMATED MATERIAL
HANDLING SYSTEM" filed on Aug. 31, 1999, and U.S. patent
application Ser. No. 09/130,766, now U.S. Pat. No. 6,008,095
entitled "REAL TIME DECISION MAKING SYSTEM FOR REDUCTION OF TIME
DELAYS IN AN AUTOMATED MATERIAL HANDLING SYSTEM" filed on Aug. 31,
1999, which are incorporated herein by reference.
Claims
What is claimed is:
1. A method for routing material in a computer controlled
manufacturing environment having a plurality of alternate locations
for a plurality of manufacturing processes, wherein at least two
objects meet at a junction of at least two routes, the method
comprising: establishing a distribution of events indicative of the
alternate locations at which material is processed; in response to
establishing the distribution of events, formulating a routing
decision for the at least two objects; determining an interval at
which to re-establish the distribution of events; in response to
determining the interval, re-establishing the distribution of
events and re-formulating a routing decision for the two
objects.
2. The method of claim 1, wherein the interval is determined using
at least one of: a predetermined time interval, a change in status
of an alternate location, a change in availability of at least one
of the at least two objects, a manual process change, and a
malfunction in the process.
3. The method of claim 1, wherein the alternate locations include a
plurality of alternate robots, a plurality of alternate transfer
units, a plurality of material cassettes, and a plurality of
stocker units.
4. The method of claim 1, wherein the objects include a
semiconductor wafer and a semiconductor wafer cassette.
5. The method of claim 1, wherein formulating a routing decision
includes formulating as a function of at least one of: the priority
level associated with material needing transfer, process data
regarding other material in the manufacturing environment, a
process malfunction, an equipment malfunction, the availability of
a material cassette, the need for a material cassette for transfer
of a subsequent material transfer, and a scheduled equipment outage
at a manufacturing process location.
6. The method of claim 1, wherein formulating a routing decision
includes formulating to achieve route efficiency.
7. A system for routing material in a computer controlled
manufacturing environment having a plurality of alternate locations
for a plurality of manufacturing processes, wherein at least two
objects meet at a junction of at least two routes, the system
comprising: a computer arrangement adapted to establish a
distribution of events indicative of the alternate locations,
formulate a routing decision for the at least two objects in
response to the established distribution of events, determine an
interval at which to re-establish the distribution of events, and
re-formulate a routing decision for the at least two objects in
response to the re-established distribution of events; and a
plurality of transport arrangements adapted to transport the at
least two objects via the at least two routes.
8. A system according to claim 7, wherein the plurality of
transport arrangements comprise: a plurality of cassettes adapted
to carry material; a plurality of stocking units adapted to stock
material; a plurality of transfer units adapted to receive and
deliver material; and a plurality of robots adapted to deliver the
material between the stocking units and the transfer units.
9. The system of claim 8, wherein the computer arrangement is
adapted to monitor the location of each cassette and determine
which type of material is capable of use with each cassette.
10. The system of claim 7, wherein the computer arrangement is
further adapted to control the plurality of transport
arrangements.
11. The system of claim 7, wherein the computer arrangement is
further adapted to monitor changes to the manufacturing environment
and, responsive to the monitored changes, re-establish the
distribution of events indicative of the alternate locations.
12. A system for routing material in a computer controlled
manufacturing environment having a plurality of alternate locations
for a plurality of manufacturing processes, wherein at least two
objects meet at a junction of at least two routes, the system
comprising: means for establishing a distribution of events
indicative of the alternate locations at which material is
processed; means for formulating a routing decision for the at
least two objects in response to an established distribution of
events; means for determining an interval at which to re-establish
the distribution of events; and means for re-establishing the
distribution of events in response to determining an interval.
13. A method for transferring material across a junction via at
least one route on each side of the junction in a computer
controlled manufacturing environment having a plurality of
alternate locations for a plurality of manufacturing processes,
wherein the material is transported in cassettes at each side of
the junction, the method comprising: recording events indicative of
the alternate locations at which material is processed; generating
ranges of relative probability figures for the respective alternate
locations as a function of the events from the recording step;
identifying a next manufacturing process step for the material for
transfer; generating a random probability; selecting one of a
plurality of alternate junctions and one of a plurality of
alternate cassettes via which to route the material for the next
manufacturing process step, the one junction and one cassette
having a range of probability figures that includes the random
probability; and transferring the material via a first cassette to
the selected one of the plurality of alternate junctions and
transferring the material to the one of the plurality of alternate
cassettes.
14. The method of claim 13, wherein the alternate locations include
a plurality of junctions, a plurality of transfer robots, a
plurality of cassettes, a plurality of stockers, and a plurality of
transfer units.
15. The method of claim 13, wherein recording events indicative of
the alternate locations at which material is processed includes
recording the type of material at each of the alternate
locations.
16. The method of claim 13, wherein selecting one of a plurality of
alternate cassettes includes selecting a cassette that is located
at the junction.
17. The method of claim 13, wherein selecting one of a plurality of
alternate cassettes includes selecting a cassette that is located
away from the junction, further comprising transferring the
cassette to the junction.
18. The method of claim 13, wherein recording events includes
recording at least one of: the priority level associated with
material needing transfer, process data regarding other material in
the manufacturing environment, the availability of process
locations, a process malfunction, an equipment malfunction, and a
scheduled equipment outage at a manufacturing process location.
19. The method of claim 13, wherein the first cassette is further
used in another transfer process.
20. A method for routing material in a computer controlled
manufacturing environment having a plurality of alternate locations
for a plurality of manufacturing processes, wherein at least two
objects meet at a junction of at least two routes, the method
comprising: establishing a distribution of events indicative of the
alternate locations at which material is processed; in response to
establishing the distribution of events, formulating a routing
decision for the at least two objects including adapting a timer to
count-down responsive to the plurality of manufacturing processes;
defining a threshold probability level as a function of the
plurality of manufacturing processes; determining a probability as
a function of the established distribution of events and the
plurality of manufacturing processes; and responsive to the
determined probability reaching the threshold probability level,
formulating the routing decision; determining an interval at which
to re-establish the distribution of events; in response to
determining the interval, re-establishing the distribution of
events and re-formulating a routing decision for at least one of
the two objects.
Description
FIELD OF THE INVENTION
The present invention generally relates to automated material
handling systems and, more particularly, to systems and methods for
scheduling automated work-cell material handling systems.
BACKGROUND OF THE INVENTION
Automated material handling systems are used in a variety of
industries to move various materials from one location to a another
location. Semiconductor fabrication facilities, in particular,
commonly employ automated material handling systems for fabricating
integrated circuits on semiconductor wafers.
A conventional semiconductor fabrication plant typically includes
multiple fabrication areas or bays interconnected by a path, such
as a conveyor belt. Each bay generally includes the requisite
fabrication tools (interconnected by a subpath) to process
semiconductor wafers for a particular purpose, such as
photolithography, chemical-mechanical polishing, or chemical vapor
deposition, for example. Material stockers or stocking tools
generally lie about the plant and store semiconductor wafers
waiting to be processed. The wafers are usually stored in cassettes
each of which typically hold up to 25 wafers. Each material stocker
typically services two or more bays and can hold hundreds of
cassettes.
The semiconductor fabrication plant, including the bays, material
stockers and the interconnecting path, typically operates under
control of a distributed computer system running a factory
management program, such as WorkStream Open sold by Consilium, Inc.
In this environment, the automated material handling system may
conceptually include the cassettes, the transportation system
(e.g., paths) and control system (e.g., the distributed computer
system).
A typical semiconductor fabrication plant, such as the one
described above, is capable of processing thousands of wafers at
any given time. The wafers are typically divided into lots which
undergo different processing sequences. Each processing sequence
typically includes a number of processing steps, each defined by a
process specification. In order to manage the transportation
required for processing such a large number of wafers through
various processing steps, manufacturers commonly employ transfer
systems. The efficiency of the transfer systems is important for
maintaining the efficiency of the overall manufacturing process and
keeping the manufacturing cost of the wafers at as low as possible.
Many present material handling systems, however, exhibit excess
movement of materials through the system which negatively impacts
manufacturing throughput.
Semiconductor manufacturers compete in a highly competitive and
capital-intensive industry. A state-of-the-art semiconductor
fabrication plant typically includes hundreds of different
fabrication tools and can cost $1 billion or more. New plants can
also become obsolete relatively quickly as the dimensions of
semiconductor devices decrease. Consequently, to manufacture a
cost-effective competitive product, semiconductor manufacturers
continually seek to increase the throughput and yield of
semiconductor wafers.
SUMMARY OF THE INVENTION
The present invention is directed to a method and system for
scheduling a work-cell transfer system in automated material
handling systems.
According to an example embodiment of the present invention, a
computer controlled manufacturing environment has a plurality of
alternate locations for a plurality of manufacturing processes. At
least two objects meet at a junction of at least two routes. A
distribution of events indicative of the alternate locations at
which material is processed is established, and a routing decision
for the at least two objects is formulated. An interval at which to
re-establish the distribution of events is determined, the
distribution of events is re-established, and a routing decision is
re-formulated for at least one of the two objects.
According to another example embodiment of the present invention, a
system is arranged for routing material in a computer controlled
manufacturing environment having a plurality of alternate locations
for a plurality of manufacturing processes. At least two objects
meet at a junction of at least two routes. A computer arrangement
is adapted to establish a distribution of events indicative of the
alternate locations, formulate a routing decision for the at least
two objects in response to the established distribution of events,
determine an interval at which to re-establish the distribution of
events, and re-formulate a routing decision for the at least two
objects in response to the re-established distribution of events. A
plurality of transport arrangements are adapted to transport the at
least two objects via the at least two routes.
In another example embodiment, the present invention includes a
method for transferring material across a junction via at least one
route on each side of the junction. The material is transported in
cassettes at each side of the junction and in a computer controlled
manufacturing environment having a plurality of alternate locations
for a plurality of manufacturing processes. Ranges of relative
probability figures are generated for the respective alternate
locations as a function of the events from the recording step, and
a next manufacturing process step for the material for transfer is
identified. A random probability is generated and one of a
plurality of alternate junctions and one of a plurality of
alternate cassettes is selected. The material for the next
manufacturing process step is routed via the junction and cassette,
wherein the junction and cassette have a range of probability
figures that includes the random probability. The material is then
transferred via a first cassette to the selected one of the
plurality of alternate junctions and subsequently to the one of the
plurality of alternate cassettes.
The above summary of the present invention is not intended to
describe each illustrated embodiment or every implementation of the
present invention. The figures in the detailed description which
follow more particularly exemplify these embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be more completely understood in consideration of
the following detailed description of various embodiments of the
invention in connection with the accompanying drawings, in
which:
FIG. 1 is a schematic diagram depicting the architecture of a
manufacturing control system, according to an example embodiment of
the present invention; and
FIG. 2 is a diagram showing the physical layout of a manufacturing
work cell, according to an example embodiment of the present
invention.
While the invention is amenable to various modifications and
alternative forms, specifics thereof have been shown by way of
example in the drawings and will be described in detail. It should
be understood, however, that the intention is not to limit the
invention to the particular embodiments described. On the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
DETAILED DESCRIPTION
According to an example embodiment, the present invention is
directed to a new and efficient method for bringing at least two
objects together from independent locations via separate routes in
a computer controlled manufacturing environment. Using the
computer, the probabilities for pickup and delivery of each of the
two objects are determined and used to determine an efficient
manner in which to bring the objects together via the separate
routes. For example, the at least two objects may include a
semiconductor wafer and a semiconductor wafer cassette, each
initially located independently of the other. The probabilities for
delivering the wafer and cassette to a junction where the wafer is
placed in the cassette are determined via the computer arrangement.
Using the determined probabilities, the pickup and delivery of the
wafer and cassette to the junction are coordinated. In a more
particular example embodiment, the pickup and delivery is
accomplished using a transport arrangement, such as a robot.
In another example embodiment of the present invention, a computer
controlled manufacturing environment has a plurality of alternate
locations for a plurality of manufacturing processes. At least two
objects meet at a junction of at least two routes. A distribution
of events indicative of the alternate locations at which material
is processed is established, and a routing decision for the at
least two objects is formulated. An interval at which to
re-establish the distribution of events is determined, the
distribution of events is re-established, and a routing decision is
re-formulated for at least one of the two objects. Using this
example embodiment, the efficiency of the manufacturing environment
can be improved in manners including a reduction in idle time, a
reduction in the amount of material and material containers needed,
and a smaller and more controllable manufacturing environment.
In a more particular example embodiment of the present invention,
the material is transported in cassettes at each side of the
junction. The distribution of events is established by recording
events indicative of the alternate locations. A range of relative
probability figures is generated as a function of the recorded
events. A next manufacturing step is generated for the material. A
random probability is generated and one of a plurality of alternate
junctions and cassettes are selected via which to route the
material for the next manufacturing process, wherein the junction
and cassette have a range of probability figures that includes the
random probability. The material is then transferred via a first
cassette to the selected alternate junction and transferred to an
alternate cassette.
For example, FIG. 1 shows a computer controlled manufacturing
system 20 for the fabrication of integrated circuits on
semiconductor wafers. The system is adapted to establish the
distribution of events, formulate the routing decision, determine
the interval, re-establish the events and re-formulate the routing
decision. The system 20 has a central bus 22 to which the various
control elements are coupled. The language or protocol used on the
bus 22 is called ISIS and is sold by ISIS Distributed Systems. A
personal computer 24 is coupled to the bus 22 for ad hoc access to
all movement functions for the cassettes and wafers.
A distributed factory system (DFS) computer program 26 sold by
Consilium, Inc. is resident on a UNIX workstation 28. The UNIX
workstation 28 connects to the bus 22 for controlling, by use of
the DFS 26, the manufacturing process of the wafers. A database 30
for use by the DFS 26, for supplying the needed information to the
DFS 26, is also resident on the workstation 28. The database 30 is
also supplied by Consilium, Inc. with the DFS 26.
The DFS 26 is the newest version of Consilium's older computer
program 32, called "WorkStream Open" or "WorkStream" for short. The
program 32 is resident on a UNIX workstation 34, which is also used
to control the manufacturing process of the wafers. A database 36
for use by the WorkStream program 32, for supplying information to
the WorkStream program 32, is also resident on the workstation 34.
The database 36 is supplied by Consilium, Inc. with the WorkStream
program 32.
Example tools 38a through 38p used in the manufacturing process of
the semiconductor wafers are coupled to the bus 22 via Hewlett
Packard work stations 40a through 40p, running equipment interface
("EI") programs 41a through 41p, respectively. The equipment
interface programs 41a through 41p function as translators between
the language of the tools and the ISIS language of the bus 22.
Stocker manager (SM) computers 42a through 42d are coupled to the
bus 22 via Hewlett Packard work stations 44a through 44d, running
equipment interface ("EI") programs 45a through 45d, respectively.
The SMs 43a-d of computers 42a-d are coupled to the EIs 45a-d of
computers 44a via a SECS II protocol and RS232 connections between
computers 42a-d and 44a-d, respectively.
In an example embodiment, each stocker 46a-d is controlled by a SM
computer 42a-d. The computers 42a-42d run Sunsoft's "Interactive
UNIX" as an operating system, and SM application programs 43a
through 43d, licensed by Daifuku. The computers 42a-42d are
inter-coupled to each other via an Ethernet.
The CFM program 48 on computer 47 is coupled to and controls all
the SM computers 42a-42d. Computer 47 is also coupled to the bus
22. The CFM program 48 functions as a global database manager for
the SMs 43a-d and passes misdirected messages to the correct
object.
The computers 42a-42d schedule movement of the cassettes in and out
of material stockers and other locations, and keep track of all the
cassettes in each stocker as well as the lot numbers assigned to
each stocker. Four material stockers 46a through 46d connect to the
personal computers 42a-42d for storing empty cassettes and
cassettes of wafers.
Note that SMs 43a-d are shown in a distributed environment. In an
alternate embodiment, a single centralized SM can be configured and
arranged to control all the stockers. Selection of a distributed
versus a centralized SM depends upon particular manufacturing
requirements.
When a lot is entered into a stocker, stocker 46a, for example, the
cassette identifier containing the lot is read and the information
is forwarded from the SM 43a to the appropriate one of DFS
computers 28 or 34 via EI 45a. The DFS then looks up the next
operation and destination for the lot. Note that present DFSs, as
exemplified by DFS 26 and WorkStream 32, only identify a single
next destination and operation. The next operation and destination
are forwarded to the SM 43a.
In accordance with the present invention, when an example SM 43a
receives the next operation and destination, the SM 43a performs
its own selection of a destination (stocker) where there are
multiple destinations at which the designated operation may be
performed. A record is kept of destinations (stockers) from which
lots are removed for performing respective operations. As a
function of the record of removal events, SMs 43a-d select next
destinations. For instance, the next destination can be selected
according to the most likely destinations at which lots will be
removed for performing the operations, or according to the most
likely destinations where a corresponding part will be delivered,
such as a wafer cassette for a wafer. The keeping of records and
the selection of next destinations may, for example, be repeated at
various intervals in the manufacturing process.
Two wafer ID servers (WIDS) 49a, 49b, which are computer programs
resident on UNIX workstations 50a, 50b, respectively, maintain
information such as regarding the wafers and wafer cassettes in
WIDS databases 56a, 56b respectively. Databases 56a, 56b are
resident on UNIX workstations 54a, 54b, respectively. The WIDS 49a,
49b maintain in the databases 56a, 56b information such as wafer
IDs, positions of the wafers within the cassettes, and the process
steps through which each wafer passes. The WIDS database 56a is a
backup database to the WIDS database 56b, for providing
redundancy.
Wafer sorters (WSR) 51a, 51b are tools that move wafers within
and/or between cassettes. They also verify the identity of wafers
and cassettes, sort wafers, and split and merge lots. The WSR 51a,
51b are coupled to the bus 22 via WSR equipment interface programs
52a, 52b, respectively, resident on Hewlett Packard workstations
53a, 53b, respectively. The WSR equipment interface programs 52a,
52b act as translators between the language of the WSR 51a, 51b and
the ISIS language of the bus 22.
Two workstations 60a, 60b are coupled to the bus 22 and to the
workstations 54a, 54b, for access to material movement server (MMS)
databases 62a, 62b. The two MMS databases 62a, 62b contain
information such as the original cassette IDs, the colors of the
cassettes (zone restrictions), cassette tags, lots-to-tags mapping,
and configuration information regarding the stockers 46. The two
MMS databases 62a, 62b are also used to validate the cassette Ids.
In an alternate embodiment, the two MMS databases 62a, 62b are
extensions of the Workstream database 36.
The two MMS programs 64a, 64b mesh the two WIDS 49a, 49b and the
two MMS databases 62a, 62b with the DFS program 26 and with the
WorkStream program 32, using the ISIS bus protocol. The MMS
programs 64a, 64b allow a requester to retrieve a cassette tag or
ID for a given lot ID, or retrieve a lot ID for a given cassette
ID.
The MMS programs 64a, 64b provide facilities for accessing lot and
cassette data for shop floor control, provide protocols to external
entities to facilitate movement of the wafers on the shop floor,
provide user interfaces for ad-hoc use by operators, and are
sensitive to time-out values when interacting with the equipment
interfaces 45a through 45d.
In connection with the present invention, and in reference to FIG.
1, please see U.S. patent application Ser. No. 08/990,059, now U.S.
Pat. No. 6,108,585 filed on Dec. 12, 1997 for a more detailed
description of an example embodiment for recording, maintaining,
and using data in regard to the processing of manufacturing
material. In particular, the '059 application is relevant to the
generation of ranges of relative probability figures and a random
probability, and to the selection of alternate manufacturing routes
using the probabilities.
According to another example embodiment of the present invention,
FIG. 2 shows an example schematic arrangement 200 for routing
manufacturing material across a barrier 240. The barrier 240 may
include items such as a wall, a curtain, a door, or a window. A
plurality of material stockers 210-213 hold material for processing
in at least one cassette. When the material is needed for
processing, a first one of the plurality of robots 220 or 221 is
used to retrieve the material from one of the stockers 210 or 211
at a first side of the barrier 240 and deliver it to one of the
transfer units 230 or 231 at the barrier 240. The transfer units
are shown with two stacked vertically at each location, with one
hidden transfer unit below each showing unit. The transfer unit
transfers the material across the barrier 240 where the material is
retrieved by a second one of the plurality of robots 222 or 223 at
the second side of the barrier 240. The second robot transfers the
material to one of the stockers 212 or 213 at the second side of
the barrier 240. Each of the transfers is coordinated via computer
arrangement 250 for optimizing the efficiency of the delivery of
material, using the methods described herein.
In another example embodiment of the present invention, and using
the computer arrangement for scheduling and controlling the
material movement, the material is initially held in a first
cassette and the robot 220 delivers the first cassette holding the
material from stocker 210 at a first side of the barrier 240 to the
transfer unit 230. A second cassette, initially empty, is brought
to the transfer unit 230 at the second side of the barrier 240.
Alternatively, the second cassette may have been held at the
transfer unit from a previous process. The computer arrangement 250
may be used to ensure that the empty cassette is proper for the
material to be transferred. The transfer unit 230 empties the first
cassette and transfers the material to the second cassette. The
first cassette, now empty, may be made available for additional
processing of other material, or may be returned back to one of the
stockers 210 or 211 at the first side of the barrier 240 by the
robot 220. A robot 222 from the second side of the barrier 240 is
used to retrieve the material, now in the second cassette, and
deliver the second cassette to another stocker 212 at the second
side of the barrier 240.
In another example embodiment of the present invention, the robots,
transfer units, and stockers are also coupled to the central bus 22
of FIG. 1 for use with the manufacturing system 20. Status
information is delivered via the bus 22 for use by the various
processors for making decisions regarding the manufacturing
process, such as which robot to use, or whether to leave an empty
cassette at a transfer unit.
The interval used for establishing a distribution of events and
making decisions for routing in the manufacturing process can be
determined using a variety of information. For example, the
interval may be a simple time interval set prior to beginning the
routing process. However, if a particular piece of equipment, such
as a stocker, a transfer unit, or a robot goes out of service, the
routing decision incorporates that information as an interval and
chooses alternative routes for the material. The interval also may
include a change in the availability of material for processing, a
malfunction, or a manual change in the process.
In another example embodiment of the present invention, the
formulation of the routing decision can take one or more outside
conditions into consideration. For example, the material for
transfer may include material of different priority levels. The
higher priority level material may take precedent over lower
priority material, even if using that precedent means a drop in
efficiency. Other conditions may include process data regarding
other material in the manufacturing environment, a process or
equipment malfunction, the length of the route to the process, a
preventive maintenance shutdown, material handler availability
(such as a cassette), or the need for a cassette for another
subsequent material transfer. The computer arrangement can be used
to monitor these and other conditions.
The outside conditions can also be used as part of the formulation
of the probability of the need for a material transfer, which can
be used to formulate the routing decision. For example, a
count-down timer can be used as a basis for a trigger for
delivering objects, such as manufacturing materials or material
containers. The timer is based on the operation of the various
manufacturing processes. As the processes proceed, the probability
that a material transfer is needed increases. Historical data
including the time that it takes a manufacturing process to finish,
along with an established distribution of events associated with
the manufacturing process, can be used in determining the
probability. If the process malfunctions, the probability does not
change until the process resumes. For instance, if the process
cannot proceed, the probability may be reduced or re-started as a
new process is begun. Alternatively, if the process cannot proceed
and new material is necessary to resume processing, the probability
may be increased to reflect the immediate or impending need for
additional material.
Using historical data, such as the time it takes for objects to be
transferred and the time it takes for a manufacturing process to
finish, a threshold probability for the need for material is
defined. As the timer counts down, the probability that a material
transfer is needed approaches the threshold probability. When the
probability reaches the threshold, the routing decision is
made.
Another advantage of using a timer in formulating a routing
decision involves the determination of which of several
manufacturing processes to prepare a material transfer for. For
example, in a semiconductor wafer manufacturing process, there may
be four empty wafer cassettes in a pool. When more than four
processes will need a material transfer, timers for each process
and related probabilities generated can be used to make a decision
as to where to route each of the empty wafer cassettes. The
processes having one of the four highest probabilities will be sent
an empty wafer carrier first, based upon which reach a threshold
probability level first.
While the present invention has been described with reference to
several particular example embodiments, those skilled in the art
will recognize that many changes may be made thereto without
departing from the spirit and scope of the present invention, which
is set forth in the following claims.
* * * * *